Support gsutil getting creds from the metadata server.
Support gsutil getting creds from the metadata server.

Add response for '/computeMetadata/v1/instance/service-accounts'.

Tested by createing a robot in a VM and running
kubectl run --image=gcr.io/cloud-builders/gsutil gsutil -- ls gs://xxx-robot
kubectl logs gsutil-86b97b74df-br878
gs://xxx-robot/install_k8s_on_robot.sh
gs://xxx-robot/setup_robot.sh
gs://xxx-robot/setup_robot_image_reference.txt

Change-Id: Iae74566aaff7f55193079130540bbb419678a198
GitOrigin-RevId: 3392b8f64eb4a781c6ae867acf15998eb4c0f691

googlecloudrobotics/core,googlecloudrobotics/core,googlecloudrobotics/core